Things evolved since I posted this bug repport , and now a middle-click button or a middle-mouse wheel event is succesfully performed by the new xorg/xinput wacom thing.

But, there is still a problem with Mypaint ; I need to have the stylus up at 5cm of the tablet to can perform the buttons... With stylus on the canvas, it's like if they don't work or have any effect...
On Terminal :
They are blocked when :
device change: Wacom Cintiq 21UX stylus <enum GDK_SOURCE_PEN of type GdkInputSource>
and works when :
device change: Core Pointer <enum GDK_SOURCE_MOUSE of type GdkInputSource>

The same effect is also observed on Gimp 2.6.10 and Krita ( but touchstrip for zoom work on Krita without have to release the stylus )

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